WASHINGTON — Senators on Wednesday approved a plan to double the number of officers along the U.S.-Mexico border, a key concession to Republicans who plan to join with Democrats in supporting a comprehensive immigration measure this week.At the same time, House Republicans signaled that there will be no quick resolution to the monthslong debate over the nation's immigration laws, regardless of what happens in the Senate.By a vote of 69-29, senators amended the immigration bill to include provisions that would double the size of the U.S. Border Patrol along the Mexican border, require the construction of 700 miles of fencing along the southern border and authorize the use of new radar and unmanned aerial drones to track illegal border crossings."Americans want immigration reform, but they want border security first," said Sen. John Hoeven, R-N.D., a key backer of the amendment.Supporters of the comprehensive immigration bill, led by the bipartisan "Gang of Eight," hope to hold a final vote this evening. Sen. Marco Rubio, R-Fla., an author of the bipartisan bill being debated, took to the Senate floor to explain — and at times defend — his involvement in the largest immigration overhaul in decades.In addition to frustrating many conservatives and tea party activists, Rubio split with his mentor — former Sen. Jim DeMint of South Carolina — on the topic of immigration."I got involved in this issue for one simple reason," Rubio said. "I ran for office to try and fix things that are hurting this special country, and in the end, that's what this is about for me — trying to fix a serious problem that faces America."